What is voltage drop?
The voltage lost as current passes through the cable, connectors and transformers of a loop — and the reason the CCR must supply more than the lamps alone need.
Voltage drop is the voltage lost as current flows through any resistance in a circuit. Every metre of cable, every connector and every isolation transformer has some resistance or impedance, so each one takes a share of the voltage equal to the current times its resistance.
In a series airfield lighting loop the same current passes through every section, so the drops add up (arithmetically for the resistive parts, as vectors where transformer reactance matters). The CCR must supply the sum of all of them, which is why long runs, thin cable and poor joints raise the voltage the CCR has to deliver. The lamps are the useful load: the voltage across each isolation transformer is mostly lamp power divided by loop current, and the CCR supplies that on top of the cable and connector drops. Drop is normal, but a rising drop over time points to corrosion, damaged cable or failing connectors.
- Take the current. Use the current of the brightness step being checked.
- Find each resistance. Use the cable length and its resistance per km, plus connector and transformer values.
- Multiply and add. Drop = I × R for each part; the total is the voltage lost in the loop.
Frequently Asked Questions
Q1What is voltage drop?
The voltage lost as current flows through the resistance or impedance of a conductor or component. It equals the current multiplied by that resistance (V = I x R), and it is dissipated as heat rather than delivered to the lights.
Q2Why does voltage drop matter in a series lighting circuit?
The CCR has to supply the sum of every drop in the loop. Long or undersized cable adds drop, which raises the output voltage the CCR needs to hold the set current and can exceed its rating.
Q3How do I calculate the drop across a cable?
Multiply the circuit current by the cable resistance for the length used. Using a resistance per kilometre from the cable data sheet, drop = I x (resistance per km x length in km).
Q4Does voltage drop dim the lights at the end of the circuit?
No. In a series circuit the same current flows through every fixture, so brightness does not fall along the loop the way it can in a parallel supply. The cost of drop is a higher required CCR voltage, not uneven light.
Q5What causes extra voltage drop in service?
Corroded or poorly made connectors, damaged cable, water ingress and ageing transformers all add resistance, so the drop and the voltage the CCR needs both creep upward over time.
Q6Is voltage drop the same as a fault?
No. Drop is normal and expected in any loaded circuit. A sudden or large change from the commissioned value is the warning sign, and should be investigated with insulation and continuity tests.
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